Cross-phase modulation (XPM) means that the signal phase in one channel is modulated by fluctuations of optical intensity of another channel. It originates from variation in refractivity of an optical fiber medium with variation in intensity of an optical field. Since there are a great number of channels in a WDM optical communication system, intensity of an optical field in the optical fiber medium randomly fluctuates, thereby applying phase noises to any channel. Such phase noises caused by cross-phase modulation are an important source leading to system cost. Because phase variation of a channel caused by cross-phase modulation is random, its characteristics are usually expressed by statistical amounts (such as autocorrelation functions). Statistical characteristics of XPM phase noises are related to many factors such as system configuration, signal power and modulation mode, and these amounts vary with topological structure, time and environment of the communication system. It appears to be very necessary to have an XPM monitoring apparatus capable of operating online, as it is conducive to such operations as compensation of phase noises, estimation of channel characteristics and optimization of the system, etc.
Currently available technologies for monitoring XPM phase noises are based on spectral measurement of signals. In “Cross-phase modulation in fiber links with multiple optical amplifiers and dispersion compensators” (JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 14, NO. 3, MARCH 1996), Ting-Kuang Chiang et al. the amplitude of phase variation generated by a sine intensity modulated adjacent channel to an observation channel is measured, but the measuring method does not apply to non-sine modulated adjacent channel, so that its application is restricted. In “Analysis and measurement of root-mean-squared bandwidth of cross-phase-modulation-induced spectral broadening” (IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 9, SEPTEMBER 1999), Keang-Po Ho et al. root-mean-squared values of spectral broadening bandwidths induced by XPM phase noises are measured. However, none of the aforementioned monitoring technologies can obtain statistical characteristics of XPM-induced phase noises in real time.